28.9 Seconds since the Epoch

If you precede a number with ‘@’, it represents an internal time
stamp as a count of seconds. The number can contain an internal
decimal point (either ‘.’ or ‘,’); any excess precision not
supported by the internal representation is truncated toward minus
infinity. Such a number cannot be combined with any other date
item, as it specifies a complete time stamp.

Internally, computer times are represented as a count of seconds since
an epoch—a well-defined point of time. On GNU and
POSIX systems, the epoch is 1970-01-01 00:00:00 UTC, so
‘@0’ represents this time, ‘@1’ represents 1970-01-01
00:00:01 UTC, and so forth. GNU and most other
POSIX-compliant systems support such times as an extension
to POSIX, using negative counts, so that ‘@-1’
represents 1969-12-31 23:59:59 UTC.

Traditional Unix systems count seconds with 32-bit two’s-complement
integers and can represent times from 1901-12-13 20:45:52 through
2038-01-19 03:14:07 UTC. More modern systems use 64-bit counts
of seconds with nanosecond subcounts, and can represent all the times
in the known lifetime of the universe to a resolution of 1 nanosecond.

On most hosts, these counts ignore the presence of leap seconds.
For example, on most hosts ‘@915148799’ represents 1998-12-31
23:59:59 UTC, ‘@915148800’ represents 1999-01-01 00:00:00
UTC, and there is no way to represent the intervening leap second
1998-12-31 23:59:60 UTC.